Pyruvic acid proton and hydrogen transfer reactions in clusters

Abstract

We investigate ion chemistry in pyruvic acid (PA) clusters in a molecular beam experiment. We generate two types of species, isolated (PA)_N clusters and clusters deposited on large water clusters (ice nanoparticles) (PA)_N·(H₂O)_M, ≈ 390, and follow their chemistry after either 70 eV electron ionization (EI) or 193 nm UV photoionization (PI). In the (PA)_N clusters, where the ionization starts with a PA molecule, both the EI and PI yield essentially the same ions: nominally (PA)_nH_k⁺, k = 1,2,3,…. Based on quantum chemical calculations, we suggest that several proton or hydrogen transfer reactions take place within a reaction cascade, with the hydrogen atoms stemming from other PA molecules. When a proton or hydrogen atom is transferred, the resulting [PA–H]˙ radical decomposes to CH₃CO˙ and CO₂ in an exothermic reaction. On the other hand, the EI and PI show entirely different patterns on nanoices: the EI proceeds mostly via water ionization yielding protonated water clusters (H₂O)_mH⁺ and, in most cases, PA molecules evaporate. The PI of pyruvic acid on nanoices exhibits essentially the same ion-chemistry as the ionization of (PA)_N clusters, demonstrating also that the individually adsorbed PA molecules coagulate on nanoices. Our results show that ionized pyruvic acid might act both as donor and acceptor of protons or hydrogen atoms, with the proton/hydrogen donation being irreversible due to decomposition of the [PA–H]˙ radical.